Well you could get a faster disk, or make a raid 0 array to increase data rates.........
as far a video cards I would definitely go with this one
http://ati.com/products/fireglx2-256t/index.html
I know when I was doing CAD a few years ago I loved dual displays, and even now when I play in Photoshop it would be very nice....
to increase speeds you have to first know what is the slow point in the system, if the cpu usage high - if so go dual cpu
if its slow to render a video card upgrade may be needed
if it just takes a while to read and write to the hdd, go to a scsi raid array, or at least some 10k rpm SATA drives in an array....